The present invention relates to probe cards which are used for probing test devices, such as integrated circuits on a wafer, and, in particular, relates to probe cards that are suitable for use in measuring ultra-low currents
Typically, in the construction of a probe card, a dielectric board is used as a base. A plurality of probing devices are mounted in radial arrangement about an opening in the board so that the probing elements of these devices, which may, for example, comprise slender conductive needles, terminate below the opening in a pattern suitable for probing the contact sites of the test device. The probing devices are individually connected to the respective channels of a test instrument by a plurality of interconnecting lines, where the portion of each line that extends between the corresponding probing device and the outer edge of the dielectric board may comprise an interconnecting cable or a conductive trace pattern formed directly on the board. In one conventional type of setup where the test devices are integrated circuits formed on a semiconductive wafer, the probe card is mounted by means of a supporting rig or test head above the wafer, and a support beneath the wafer moves the wafer so that each device thereon is consecutively brought into contact with the needles or probing elements of the probe card.
With particular regard to probe cards that are specially adapted for use in measuring ultra-low currents (down to the femtoamp region or lower), probe card designers have been concerned with developing techniques for eliminating or at least reducing the effects of leakage currents, which are unwanted currents that can flow into a particular cable or channel from surrounding cables or channels so as to distort the current measured in that particular cable or channel. For a given potential difference between two spaced apart conductors, the amount of leakage current that will flow between them will vary depending upon the volume resistivity of the insulating material that separates the conductors, that is, if a relatively lower-resistance insulator is used, this will result in a relatively higher leakage current. Thus, a designer of low-current probe cards will normally avoid the use of rubber-insulated single-core wires on a glass-epoxy board since rubber and glass-epoxy materials are known to be relatively low-resistance insulators through which relatively large leakage currents can flow.
One technique that has been used for suppressing interchannel leakage currents is surrounding the inner core of each lead-in wire with a cylindrical xe2x80x9cguardxe2x80x9d conductor, which conductor is maintained at the same potential as the inner core by a feedback circuit in the output channel of the test instrument. Because the voltage potentials of the outer guard conductor and the inner conductive core are made to substantially track each other, negligible leakage current will flow across the inner dielectric that separates these conductors regardless of whether the inner dielectric is made of a low- or high-resistivity material. Although leakage current can still flow between the guard conductors of the respective cables, this is typically not a problem because these guard conductors, unlike the inner conductive cores, are at low impedance. By using this guarding technique, significant improvement may be realized in the low-level current measuring capability of certain probe card designs.
To further improve low-current measurement capability, probe cards have been constructed so as to minimize leakage currents between the individual probing devices which mount the probing needles or other elements. With respect to these devices, higher-resistance insulating materials have been substituted for lower-resistance materials and additional conductive surfaces have been arranged about each device in order to perform a guarding function in relation thereto. In one type of assembly, for example, each probing device is constructed using a thin blade of ceramic material, which is a material known to have a relatively high volume resistivity. An elongate conductive trace is provided on one side of the blade to form the signal line and a backplane conductive surface is provided on the other side of the blade for guarding purposes. The probing element of this device is formed by a slender conductive needle, such as of tungsten, which extends in a cantilevered manner away from the signal trace. Such devices are commercially available, for example, from Cerprobe Corporation based in Tempe, Ariz. During assembly of the probe card, the ceramic blades are edge-mounted in radial arrangement about the opening in the card so that the needles terminate within the opening in a pattern suitable for probing the test device. The conductive backplane on each blade is connected to the guard conductor of the corresponding cable and also to corresponding conductive pad or xe2x80x9clandxe2x80x9d adjacent the opening in the probe card. In this manner each conductive path is guarded by the backplane conductor on the opposite side of the blade and by the conductive land beneath it.
It has been found, however, that even with the use of guarded cables and ceramic probing devices of the type just described, the level of undesired background current is still not sufficiently reduced as to match the capabilities of the latest generation of commercially available test instruments, which instruments are able to monitor currents down to one femtoamp or less. Thus, it was evident that other changes in probe card design were needed in order to keep pace with the technology of test instrument design.
In the latest generation of probe cards, efforts have been directed toward systematically eliminating low-resistance leakage paths within the probe card and toward designing extensive and elaborate guarding structures to surround the conductors along the signal path. For example, in one newer design, the entire glass-epoxy main board is replaced with a board of ceramic material, which material, as noted above, presents a relatively high resistance to leakage currents. In this same design, the lead-in wires are replaced by conductive signal traces formed directly on the main board, which traces extend from an outer edge of the main board to respective conductive pads that surround the board opening. Each pad, in turn, is connected to the signal path of a corresponding ceramic blade. In addition, a pair of guard traces are formed on either side of each signal trace so as to further isolate each trace against leakage currents.
In yet another of these newer designs, a main board of ceramic material is used having three-active layers to provide three dimensional guarding. Above this main board and connected thereto is a four-quadrant interface board that includes further guard structures. Between these two board assemblies is a third unit including a xe2x80x9cpogo carousel.xe2x80x9d This pogo carousel uses pogo pins to form a plurality of signal lines that interconnect the interface board and the lower main board. It will be recognized that in respect to these pogo pins, the effort to replace lower resistance insulators with higher resistance insulators has been taken to its practical limit, that is, the insulator that would normally surround the inner conductor has been removed altogether.
The probe card designs which have just been described represent the current state-of-the-art. From the foregoing examples, it will be seen that a basic concern in the art has been the suppression of interchannel leakage currents. Using these newer designs, it is possible to measure currents down to nearly the femtoamp level. However, the ceramic material used in these newer designs is relatively more expensive than the glass-epoxy material it replaces. Another problem with ceramic materials is that they are relatively susceptible to the absorption of surface contaminants such as can be deposited by the skin during handling of the probe card. These contaminants can decrease the surface resistivity of the ceramic material to a sufficient extent as to produce a substantial increase in leakage current levels. In addition, the more extensive and elaborate guarding structures that are used in these newer designs has contributed to a large increase in design and assembly costs. Based on these developments it may be anticipated that only gradual improvements in the low-current measurement capability of the cards is likely to come about, which improvements, for example, will result from increasingly more elaborate guarding systems or from further research in the area of high resistance insulative materials.
It should also be noted that there are other factors unrelated to design that can influence whether or not the potential of a particular probe card for measuring low-level currents will be fully realized. For example, unless special care is taken in assembling the probe card, it is possible for surface contaminants, such as oils and salts from the skin or residues left by solder flux, to contaminate the surface of the card and to degrade its performance (due to their ionic character, such contaminants can produce undesirable electrochemical effects) Furthermore, even assuming that the card is designed and assembled properly, the card may not be suitably connected to the test instrument or the instrument may not be properly calibrated so as to completely null out for example the effects of voltage and current offsets. In addition, the probe card or the inter-connecting lines can serve as pickup sites for ac fields, which ac fields can be rectified by the input circuit of the test instrument so as to cause errors in the indicated dc values. Thus, it is necessary to employ proper shielding procedures in respect to the probe card, the interconnecting lines and the test instrument in order to shield out these field disturbances. Due to these factors, when a new probe card design is being tested, it can be extremely difficult to isolate the causes of undesirable background current in the new design due to the numerous and possibly interacting factors that may be responsible.
In view of the foregoing, what is needed is a probe card that is capable of being used for the measurement of ultra-low level currents but yet can be inexpensively manufactured from relatively low-cost materials in accordance with a relatively straightforward assembly process.
In accordance with the present invention, the inventor has discovered that the primary problem, at least at some stage in the design, is not how best to suppress the leakage currents that flow between the different signal channels but rather how best to suppress those currents that internally arise in each cable or signal channel as a result of the triboelectric effect. In a guarded cable, triboelectric currents can arise between the guard conductor and the inner dielectric due to friction therebetween which causes free electrons to rub off the conductor and creates a charge imbalance that causes current to flow. Once the inventor recognized that this triboelectric effect might be the critical problem, he proceeded to test this insight by substituting xe2x80x9clow-noisexe2x80x9d cables for the guarded cables that had heretofore been used. These low-noise cables, which were custom-made in order to meet size constraints, made a significant difference to the low current measurement capability of the probe card. Indeed, even though these cables were used in connection with a relatively inexpensive glass-epoxy board, and even though, under conventional thinking, this type of material did not possess sufficiently high resistance to permit ultra-low current measurements, the inventor was able to achieve current measurements down to the femtoamp region. Within weeks of this discovery, the commercial value of this invention became readily apparent when measurement data taken from a prototype of the subject probe card was instrumental to a customer purchase order for two probing stations worth hundreds of thousands of dollars apiece.
It will be noted that the inventor does not claim to have discovered a new solution to the problem of the triboelectric effect. A relatively straightforward solution to this problem can be found in the field of cable technology wherein it is known how to construct a xe2x80x9clow-noisexe2x80x9d cable by using an additional layer of material between the outer conductor and the inner dielectric, which material is of suitable composition for suppressing the triboelectric effect. This layer, in particular, includes a nonmetallic portion that is physically compatible with the inner dielectric so as to be prevented from rubbing excessively against this dielectric and, on the other hand, includes a portion that is sufficiently conductive that it will immediately dissipate any charge imbalance that may be created by free electrons that have rubbed off the outer conductor. It is not claimed by the inventor that this particular solution to the triboelectric effect problem is his invention. Rather it is the recognition that this specific problem is a major source of performance degradation in the field of low-current probe card design that the inventor regards as his discovery.
In retrospect, one can speculate as to why the significance of the triboelectric effect was not recognized sooner by investigators in the art of probe card design. One possible reason is that verifying the importance of this effect is not merely a matter of replacing guarded cables with low-noise cables. As indicated, in the Background section hereinabove, traces formed directly on the main dielectric board have largely replaced guarded cables in the newer generation of probe card designs, so that in order to begin with a design where this problem is amendable to a straightforward solution, one must return to an older and presumably less effective technology. Moreover, because of the non-design related factors specified in the Background section, one of ordinary skill who assembled and then tested a probe card that included low-noise cables would not necessarily detect the superior capability of this probe card for low current measurements. For example, surface contaminants deposited on the probe card during its assembly might raise the background level of current to a sufficient extent that the effect of the low-noise cables is concealed. To this it may be added that the direction taken in the art of probe card design, where the focus has been on the problem of suppressing interchannel leakage currents, has provided solutions which, by happenstance, have also substantially resolved the triboelectric effect problem. These solutions, which included replacing cables with trace-like conductors on ceramic boards or using signal lines in which no insulator at all surrounds the signal conductor (as in the case of signal lines formed by pogo pins) are complicated and expensive compared to the inventor""s relatively straightforward solution to the triboelectric effect problem. However, the indirect and mitigating effect of these alternative solutions, which served to conceal the problem, does help to explain why a more direct solution to the triboelectric effect problem was overlooked even by sophisticated designers of state-of-the-art probe cards.
In accordance, then, with the present invention, a probe card is provided for use in measuring ultra-low currents, which probe card includes a dielectric board, a plurality of probing devices that are edge-mounted in radial arrangement about an opening in the board, and a plurality of cables for connecting each probing device to a corresponding channel of a test instrument. These cables are of suitable construction to be used in a guarded mode, that is, they include an outer conductor that surrounds the inner conductor or core of the cable, which outer conductor can be used as a guard conductor in relation to the inner conductor. Furthermore, these cables include an inner layer of material between the outer conductor and the underlying inner dielectric, which layer is of suitable composition for reducing triboelectric current generation between the inner dielectric and the outer conductor to less than that which would occur were the inner dielectric and the outer conductor to directly adjoin each other.
In accordance with the foregoing construction, a probe card is provided in which a significant source of background current is suppressed in a relatively straightforward manner thereby eliminating the need for providing complicated and expensive structures to suppress other less significant sources in order to achieve the capability of measuring ultra-low currents. This and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.